[Xrystal]

I think the red dot on white line is the latency bar but could be like you say the GCD bar.

To disable that you want to do the following :

Go to folder :
Interface\AddOns\nUI\Layouts\Default\HUDLayouts\PlayerTarget

Open the following file in any text editor :
nUI_HUDSkin_PlayerTarget_Castbar.lua

Look for the following block of code after the comment section :

		["GCD"] =
		{
			anchor =
			{
				anchor_pt   = "BOTTOM",
				relative_to = "$parent_Casting",
				xOfs = 0,
				yOfs = 0,
			},		
			options =
			{
				enabled    = true,
				height     = 20,
				width      = 375,
				inset      = 0,
				strata     = nil,
				level      = 9,
				barColor   = { r = 0.75, g = 0.75, b = 0.75 },
				sparkColor = { r = 1, g = 0.5, b = 0.5 },
			},
		},

Simply change the highlighted section from true to false and that should turn that particular part over.